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United States Patent |
5,065,070
|
Longo
,   et al.
|
November 12, 1991
|
Sputtered scandate coatings for dispenser cathodes
Abstract
A low work function surface for a dispenser cathode structure. The cathode
structure comprising a heater and an electron emitting surface substrate
or core composed of a porous tungsten matrix impregnated with a barium
containing impregnant distributed therethrough. The structure is made by a
method in which a nanometer thick layer of scandium oxide is sputtered
onto the outermost surface of the impregnated tungsten core, or substrate,
and then oxidized by exposing the sputtered scandium oxide surface layer
to an oxygen atmosphere. The oxidized surface layer is activated by
turning on the heater, for example, to cause the release of a small
portion of the barium in the barium-containing impregnant. Some of the
released barium migrates into the scandium oxide surface layer to form a
monolayer of barium oxide on at least a portion thereof.
Inventors:
|
Longo; Robert T. (Arcadia, CA);
Barillas; Mario A. (Carson, CA);
Forman; Ralph (Rocky River, OH)
|
Assignee:
|
Hughes Aircraft Company (Los Angeles, CA)
|
Appl. No.:
|
696399 |
Filed:
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May 6, 1991 |
Current U.S. Class: |
313/346DC |
Intern'l Class: |
H01J 001/28 |
Field of Search: |
313/346 DC,346 R
|
References Cited
U.S. Patent Documents
3121048 | Feb., 1964 | Haas | 313/346.
|
4007393 | Feb., 1977 | Van Statum et al. | 313/346.
|
4427916 | Jan., 1984 | Shroff | 313/346.
|
4625142 | Nov., 1986 | Van Esdonk et al. | 313/346.
|
4855637 | Aug., 1989 | Watanabe et al. | 313/346.
|
5006753 | Apr., 1991 | Hasker et al. | 313/346.
|
Primary Examiner: DeMeo; Palmer C.
Attorney, Agent or Firm: Gudmestad; Terje, Denson-Low; W. K.
Parent Case Text
This is a division of application Ser. No. 632,194, filed Dec. 21, 1990,
now U.S. Pat. No. 5,041,757.
Claims
What is claimed is:
1. A low work function scandate surface coating for a dispenser cathode
structure containing an outer surface substrate composed of porous
tungsten impregnated with a barium containing impregnant distributed
therethrough, the surface coating comprising a 1 to 30 nanometer thick
layer of scandium oxide deposited onto the outermost portion of the
substrate, the scandium oxide surface coating further comprising an
activating amount of barium oxide.
2. The cathode coating of claim 1 wherein the scandium oxide coating layer
has a thickness in the range between about 5 and 20 nanometers.
3. The cathode coating of claim 1 wherein the scandium oxide coating layer
has a thickness in the range between about 8 and 15 nanometers.
4. The cathode coating of claim 1, wherein the scandium oxide coating layer
has a thickness in the range between about 10 and 12 nanometers.
5. The cathode coating of claim 1 wherein the surface coating is deposited
by sputtering.
6. The cathode coating of claim 1, wherein the porous tungsten substrate
has a density of 80.+-.10 percent of its theoretical density and the
barium containing impregnant is barium-calcium aluminate.
Description
BACKGROUND
The present invention relates to low work surface function coatings for
high current density dispenser cathodes and more particularly to
barium-activated scandium oxide surface coatings for such cathodes, and
methods for making same.
High current density dispenser cathodes are widely used as the electron
source in display tubes, camera tubes, oscilloscope tubes, klystrons,
transmitter tubes and the like. A characteristic of such cathodic
structures is that there is a functional separation between the electron
emissive surface and a store of emissive material which serves to produce
a sufficiently low work function of the emissive surface.
One type of dispenser cathode is a "scandate" cathode, in which the
electron emission takes place from the surface of a porous matrix of, for
example, tungsten impregnated with a barium-calcium aluminate mixture
which is distributed therethrough. In these cathodes, the surface work
function is reduced by impregnating or embedding at least the top surface
of the metal matrix with an electron emissive material comprised of
scandium oxide (Sc.sub.2 O.sub.3). When this is done, the finished product
has a lower surface work function and better long term stability, as
compared to either uncoated barium impregnated tungsten dispenser cathodes
or to dispenser cathodes coated with a mixture of osmium and ruthenium
(surface work function 1.80 to 1.85 eV).
Scandium oxide is, however, a semiconductor/insulator and, at high current
densities, its resistance to electron current flow causes significant
problems when the oxide particles are in or above the range of microns in
size. While this problem has been recognized, procedures developed to
overcome it have, so far, not proven to reliably produce high quality
materials at a relatively reasonable cost. For example, Hasker et al.
disclose in U.S. Pat. No. 4,594,220, a multistep method which involves
sintering a compressed mixture of tungsten and scandium hydride powder
previously deposited onto a porous barium and scandium oxide-containing
porous tungsten substrate. In another method, disclosed by Hitachi
Corporation in U.K. Patent 2,170,950, a combination of metal (preferably
tungsten) and scandium oxide is deposited onto an conventinal scandate
type scandium oxide impregnated cathode surface by sputtering. This method
has proven to produce erratic and inconsistent results because of the
difficulty in maintaining the correct ratio of metal and scandium oxide in
the sputtered layer.
It is therefore the objective of the present invention to provide an
improved scandate cathode and a simpler method for making same.
SUMMARY OF THE INVENTION
The present invention is a low work function scandate surface for a
dispenser cathode structure and a method of making same. The cathode
comprises an outer electron emitting surface comprising a core or
substrate which is a matrix composed of a major part of porous tungsten
with minor parts of a barium-containing impregnant and, occasionally,
scandium oxide distributed therethrough, and with a nanometer thick layer
of barium activated scandium oxide being deposited on the outermost
surface thereof as the electron emitting material. The cathode structure
further comprises a heater and an insulator. All of the above listed
components are held together to form a finished cathode structure by a
sleeve or other retaining device.
The method of making the above-described cathode comprises depositing a
layer of scandium oxide onto the outermost surface of a barium impregnated
tungsten-containing substrate. The deposited oxide layer has a final
thickness in the range of between about 1 and about 30 nanometers. Any
conventional deposition method that produces the coating may be used as
long as the deposited coating has the proper thickness and composition.
Suitable methods include sputtering and evaporation chemical vapor
deposition (CVD). The deposited scandium oxide surface layer is then
exposed to an oxygen atmosphere for between about 2 and about 10 minutes
at a temperature of between about 375.degree. C. and 500.degree. C., and
an oxygen pressure of between about 10.sup.-5 and about 10.sup.-7 torr.
The oxygen exposed oxide surface layer is then activated by turning on the
cathode heater, for example, to cause the release of a small portion of
the barium in the barium impregnant and the subsequent migration of at
least some of the released barium into the scandium oxide surface layer.
This forms a monolayer of barium oxide on at least a portion of the
scandium oxide surface layer.
The work function of the activated surface, when made by the process
described above, is in the range of between about 1.5 and 1.6 eV. Further,
the binding energy of barium oxide to scandium oxide is very high, so that
the surface complex formed by this process is quite stable, even at
temperatures in excess of about 700.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of the present invention may be more
readily understood with reference to the following detailed description
taken in conjunction with the accompanying drawings, wherein like
reference numerals designate like structural elements, and in which:
FIG. 1 is an overall plan view of a typical dispenser cathode structure
containing the scandate coating of the present invention;
FIG. 2 is useful in illustrating one method in accordance with the
principles of the present invention; and
FIG. 3 is a graph showing the improvement in surface work function achieved
with a cathode structure of the present invention as compared to a
conventional M type cathode.
DETAILED DESCRIPTION
FIG. 1 illustrates a typical configuration for an electron emitting
dispenser cathode structure 10. As shown therein, the cathode structure 10
comprises an outer electron emitting surface formed of a porous tungsten
substrate 12. The porous substrate 12 is impregnated with a barium
containing electron activator distributed therethrough, preferably,
barium-calcium aluminate, and further, has a nanometer thick uppermost
layer 14 of scandium oxide deposited thereon. Also shown in FIG. 1 are a
heater 16, an alumina insulator 18 into which the heater 16 is placed, and
a retainer 20, which is typically a can or flanged sleeve and which holds
all of the constituent parts of the assembled dispenser cathode structure
10 in proper position for subsequent use. For the purposes of the
subsequent discussion, it is known that the heater 16, the insulator 18
and the retainer 20 are all more-or-less standard in the art.
In the structural embodiment shown in FIG. 1, the porous tungsten substrate
12 is typically a blank that is about one inch square by about 1/4 inches
thick. This blank is normally fabricated from tungsten powder having a
particle size in the range of between about 4.0 to about 7.5 microns in
diameter, that is compacted and fused to form a finished substrate blank
having a density which is usually about 80.+-.10% of theoretical density.
When the blank is to be used as the substrate 12 for the dispenser cathode
structure 10 of the type herein described, one widely used technique for
impregnating the porous blank with a barium containing activator is by
capillary action. Typically, this is accomplished by heating the blank to
a temperature above the melting point of a "4-1-1" barium-calcium-aluminum
oxide mixture, i.e., one having a molar ratio of about 4 parts of barium
oxide to about 1 part each of calcium oxide and aluminum oxide, and then
immersing the heated blank in this oxide mixture.
In the present invention, the uppermost layer 14 has a scandium oxide
thickness in the approximate range between 1 to 30 nanometers, preferably
between 5 and 20 nanometers, more preferably between 8 and 15 nanometers,
and most preferably between 10 and 12 nanometers. In these approximate
ranges, the Sc.sub.2 O.sub.3 thickness is so low that its resistance does
not seriously impede a high current electron flow from substrate 12. The
scandium oxide uppermost layer 14 may be deposited by a number of well
known methods, such as by chemical vapor deposition and sputtering, but is
preferably deposited by sputtering Sc.sub.2 O.sub.3 onto the uncoated,
outermost surface of the substrate 12, using an argon plasma as the
carrier. In the preferred process, as illustrated in FIG. 2, an R.F.
generator 22 is internally D.C. biased to make the target area of the
substrate 12 negative relative thereto. By so doing, a charge is prevented
from building up on an insulating scandium oxide electrode 24 employed to
deposit the uppermost layer 14 on the substrate 12. Further, O.sub.2 gas
can also be injected along with the argon to control the final product
composition.
Because of the uncertainties associated with a deposition process such as
sputtering, it is preferred that before a full production run is started,
the rate of scandium oxide deposition be determined first by running a
blank sample at a given R.F. generator 22 power level for a given time.
After measuring the thickness of the scandia layer deposited thereon, the
run time needed to deposit any given thickness of scandium oxide can be
easily established. Preferably, this time is adjusted to that required to
deposit about a 10 nanometer thick layer of scandium oxide on the
outermost surface of the tungsten-containing substrate 12. At the
conclusion of the sputtering step, to ensure that the deposited surface
layer 14 is all Sc.sub.2 O.sub.3 and not just scandium rich Sc.sub.2
O.sub.3, the scandium oxide outer surface 14 of substrate 12 is exposed to
an oxygen atmosphere for a time between about 2 and 10 minutes, preferably
between about 4 and about 7 minutes, at between about 375.degree. C. and
500.degree. C., preferably between about 400.degree. C. and about
450.degree. C., at an oxygen pressure of between about 10.sup.-5 and about
10.sup.-7 and preferably between about 1.times.10.sup.-6 and about
6.times.10.sup.-6 torr.
Following these steps, the surface 14 is activated by turning on the heater
16 and operating the dispenser cathode structure 10 as a low surface work
function electron emitting cathode. Alternatively, other means of heating
the structure to activate the surface 14 may be employed. Every time the
porous cathode structure 12 is heated, small amounts of barium are
released by a reaction of the barium-containing impregnant with the
tungsten. During such operation, at least some of the released barium will
migrate into the thin scandium oxide surface layer 14, where it forms a
monolayer of BaO on at least a portion thereof, thus completing the
activation of the low work function surface of cathode structure 10. This
barium release and migration continues throughout the effective lifetime
of the cathode structure 10, thus maintaining its low work function
surface characteristics without a significant diminution of same occurring
during this time.
The degree of improvement achieved with cathodes made by the method of the
present invention is shown in FIG. 3. This shows that at a current density
of approximately 3.75 amperes/cm.sup.2 a work surface energy value of
about 1.6 electron volts is achieved at a working temperature of about
1000.degree. K., whereas a comparable conventional standard M cathode
showed, at this same current density, a work surface energy value of about
1.9 electron volts at a working temperature in excess of 1100.degree. K.
In still other embodiments of the present invention, the barium activator
may be applied either by spraying a small amount of barium oxide onto the
upper surface of the impregnated core cathode structure 10 prior to
sputtering the scandium oxide thereon, or by cosputtering barium oxide
with the scandium oxide. Further, the tungsten substrate 12 may also have
some amount of scandium oxide mixed in prior to the sintering operation.
Regardless of how the scandium oxide surface is activated with barium
oxide, the resultant cathode structure 12 is characterized by having a low
work function surface which is further characterized by being a copious
electron emitter at relatively low operating temperatures and having a
long service lifetime.
Thus there has been described a new and improved barium-activated scandium
oxide-containing surface coating for a dispenser cathode structure and
methods for making same. It is to be understood that the above-described
embodiments are merely illustrative of some of the many other specific
embodiments which represent applications of the principles of the present
invention. Clearly, numerous and other arrangements can be readily devised
by those skilled in the art without departing from the scope of the
invention.
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